Recently, the image detector size in new video cameras being sold has been decreasing from about 1/3 of an inch in linear dimension to about 1/4 of an inch in linear dimension. This has placed demands on the development of new zoom lenses for such cameras which are simple in design and yet have a sufficiently high optical performance. As such a zoom lens, rear-focus type lenses have been known in which the entire zoom lens is formed of four lens groups, with the first lens group and the third lens group being fixed in position. Power is varied by moving the second lens group along the optical axis and the image plane is made to remain at a fixed location as the power is varied by moving the fourth lens group along the optical axis. Moreover, lenses with a high variable power ratio, as described in Japanese Laid Open Patent Applications Nos. H8-005916 and H11-194269 have been known for use as zoom lenses for video cameras.
More recently, handy electronic still cameras, so-called digital cameras, have rapidly become popular, and zoom lenses of high variable power ratio have also been used in such electronic still cameras. However, the required degree of compactness is different among video cameras and digital cameras. Therefore, the overall length of zoom lenses for these two applications is also different. Namely, the size of the digital camera is much smaller, as it is desired that such cameras be capable of being carried in a breast pocket of a garment. Thus, the overall length of a zoom lens for a digital camera must be very small. Indeed, the desired overall length of a zoom lens for a digital camera is so small that none of the above-discussed prior art zoom lenses used in video cameras is sufficiently small. Therefore a zoom lens having a shorter overall length has been desired.
In digital cameras, the resolution required of the zoom lens employed has rapidly increased as the pixel size has decreased in linear dimension to about 3-4 .mu.m. This has made the axial chromatic aberration at the telephoto end increases to the point that it is no longer of little concern.